Heretofore, the magnetic disk apparatus using aluminum as its disk substrate has been generally used. But recently, in the field of hard disk apparatus, capacity expansion and miniaturization is desired. As a method of capacity expansion, glass or ceramics are conceived to be used as the disk substrate. Such disk substrate, by its excellent surface characteristics and the like, make it easy to attain the low floatation of the head, thereby increasing the recording density.
However, the conventional magnetic disk apparatus with aluminum substrate can shield the electromagnetic coupling of a pair of top and bottom magnetic heads, which exist on each sides of the magnetic disk, but the magnetic disk apparatus with a glass or ceramic substrate cannot shield the aforesaid electromagnetic coupling of the upper and lower magnetic heads, thereby causing the crosstalk noise to increase, and the reproduced signal to deteriorate. For example, when the lower head records a 4 MHz signal while the upper head reproduces it, the measured crosstalk (at the 4 MHz output level) for the glass substrate disk 1.27 mm thick is greater than the crosstalk for the aluminum substrate disk. The crosstalk of the glass substrate disk is +8.9 dB higher than that of the aluminum substrate disk. Also, when the attenuation ratio of reproduced signal is defined as f'.sub.4M / f.sub.4M, and the ratio of crosstalk as f'.sub.1.5M / f'.sub.4M, and f.sub.4M, f'.sub.4M, and f'.sub.1.5M for glass substrate disk are measured, the ratios for the glass disk are -1.73 dB and -46.4 dB, respectively. When f.sub.4M, f'.sub.4M, and f'.sub.1.5M for aluminum substrate disk are measured, the attenuation ratio of reproduced signal, f'.sub.4M / f.sub.4M, is -0.38 dB and the ratio of crosstalk f'.sub.1.5M / f.sub.4M is -60.6 dB. It is clear from the above-mentioned features, that the glass substrate disk, when compared with the aluminum substrate disk, has considerably greater ratios of attenuation and of crosstalk. Where the f.sub.4M =reproduced signal level by the top head at 4 MHz before recorded by the bottom head, f'.sub.4M =reproduced signal level at 4 MHz by the bottom head after recorded by bottom head, and f'.sub.1.5M =reproduced signal level at 1.5 MHz by the top head after recorded by the bottom head.
The magnetic characteristics of the recording layer of the magnetic disk being used in the experiment are: EQU Hc=1.times.10.sup.3 Oe, M r t.perspectiveto.4.2.times.10.sup.-3 emu/cm.sup.2
for aluminum substrate disk, and EQU Hc.perspectiveto.1.25.times.10.sup.3 Oe, M r t 4.0.times.10.sup.-3 emu/cm.sup.2
for the glass substrate disk.
The magnetic head used has a gap length of 0.6 .mu.m, the track width of Tw.perspectiveto.10 .mu.m. In this case, the floating amount of the magnetic head is 8 .mu.. As shown above, a conventional magnetic disk apparatus using an aluminum substrate disk causes only a slight crosstalk noise problem because aluminum substrate disk can shield the electromagnetic coupling of the top and bottom magnetic heads.
However, the magnetic disk apparatus with a glass substrate disk cannot shield the electromagnetic coupling of the top and bottom magnetic heads as mentioned above, thus the crosstalk noise becomes a serious problem.
The aforementioned conventional magnetic disk apparatus with the glass substrate disk has a problem of serious crosstalk noise caused by the electromagnetic coupling of the top and bottom magnetic heads across the disk. Therefore, there is a need for an apparatus and method for reducing the crosstalk on a disk with a glass or similar substrate.